The present invention relates to lift cranes, and more particularly to connection systems for aligning sectional boom members for cranes and the like.
Large capacity lift cranes typically have elongate load supporting boom structures comprised of sectional boom members secured in end-to-end abutting relationship. Predominantly, each of the sectional boom members is made of a plurality of chords and lacing or lattice elements. The terminal end portions of each chord are generally provided with connectors of one form or another to secure abutting boom segments together and to carry compressive loads between abutting chords. Typical connectors comprise male and female lugs secured by a pin carrying compressive loads in double shear.
An example 220 foot boom may be made of a 40 foot boom butt pivotally mounted to the crane upper works, a 30 foot boom top equipped with sheaves and rigging for lifting and supporting loads, with five sectional boom members in between: one 10 feet in length, one 20 feet in length and three 40 feet in length. Such an example boom has six boom segment connections. Typically each segment has four chords, and hence four connectors, making a total of 24 connectors that must be aligned and pinned to assemble the boom.
Large capacity cranes require very large boom cross sections. As a result, even when the boom segments are laying flat on the ground, the pin connectors between the top chords are typically eight feet or higher off the ground. The rigging personnel must either move a step ladder to each pin location or stand and walk along the top of the boom to reach the top connectors.
A 40 foot long sectional boom member may weight over 5,000 lbs. Thus, an assist crane is required to lift the boom member. One rigger usually then holds the suspended boom segment in general alignment while a second rigger uses a large hammer (10 or 15 lbs.) to manually drive the pin, which typically has a long taper, into position. The pins connecting the boom segments are generally used to carry the compressive loads between chords. As a result, the pins have a tight fit, further increasing the difficulty in assembling the boom. As such, it may take three men (a crane operator and two riggers) four or more hours to assemble the example 220 foot boom. Where the crane is moved frequently, the costs to assemble and disassemble the boom may exceed the cost to lift and position the load for which the crane is used.
To carry very high loads for a high capacity crane, a typical single male lug sandwiched between two female lugs, giving a double shear connection, requires a very large pin diameter to carry the compressive loads, requiring the connectors to be very large. There are known connectors with three female lugs and two male lugs, but there is no provision for these types of boom connections to provide for any self-alignment or rotatable connection (where the boom segments can be initially connected when not axially aligned and then swung into a position where the reminder of the connections can be made) between the boom sections as the sections are assembled.
Thus, an easy, quick-connect system for boom segments that allows faster connection of the boom segments and an initial connection from a position where the boom segments are not in axial alignment would be a great improvement.